The present invention relates to a receiving system including a first equalizer associated with a first threshold detector and a device which enables the timing of a transmitting system to be recovered for said receiving system from an equalized signal and a signal obtained by thresholding.
Such a system can be included in any digital communication system, such as, amongst other possible applications, a modem, a set-top box or a television set.
The French Patent Application FR 93 10866 describes such a receiving system. The timing recovery device comprises a second-order phase-locked loop whose input is a function which depends on the shift between the clocks of the transmitting and receiving systems. The output of this loop is applied to a control device which determines the lead or lag to be applied to the clock of the receiving system.
The prior-art receiving system has some drawbacks.
First of all, the simultaneous operation of the adaptive equalizer and of the timing recovery device forms a source of instability for the receiving system. As a matter of fact, these two devices interfere with one another, particularly during the initialization phase of the communication. During this period the timing has not yet been recovered and the adaptation of the equalizer then depends notably on the error as a result of the shift between the clocks of the transmitting and receiving systems. As is shown in FIG. 1, this results in a shift (xcex4) of the impulse response (h(t)) of the equalizer filter, which in the present case is modeled in accordance with a Nyquist function having a major lobe and minor lobes, in order to compensate for this error. Since the impulse response of the filter has a limited length, its length being defined by an equalization window, it is off-centered in the equalization window, which amounts to said window being shortened. Thus, the channel is equalized poorly and the convergence of the timing recovery device is slowed down.
Moreover, the operations performed by the adaptive equalizer and by the timing recovery device require a substantial amount of calculation time, particularly during the data transmission phase.
It is an object of the invention to eliminate these drawbacks to a substantial extent by providing a stable receiving system including an effective equalizer in conjunction with a timing recovery device which ensures a rapid convergence of the receiving system clock with respect to that of the transmitting system, the receiving system also enabling calculation time to be saved during the data transmission phase.
To this end, the invention proposes a receiving system which is characterized in that said receiving system also includes a second equalizer, which is a fixed equalizer associated with a second threshold detector and a device by means of which it is possible to switch between a first mode of operation in which the equalized signal and the signal obtained by thresholding are to be supplied by the second equalizer and the second threshold detector, respectively, and a second mode of operation in which the said signals are to be supplied by the first equalizer and the first threshold detector, respectively.
Such as receiving system makes it possible to achieve that the first equalizer, i.e. the adaptive equalizer, and the timing recovery device operate independently during a first mode of operation which corresponds to the communication initialization phase. During this period the timing recovery device operates with a second equalizer which comprises a filter whose coefficients are fixed and which consequently no longer interferes with this device, thereby giving the receiving system a higher stability and enabling a more rapid convergence of the clock of the receiving system to be achieved with respect to that of the transmitting system.
The invention also proposes a receiving system which is characterized in that the first equalizer is adapted to transfer, at appropriate time intervals, coefficients of an adaptive filter to the second equalizer, the said coefficients being calculated from an equalization error equal to a difference between the signals obtained from the first equalizer and the first threshold detector.
Thus, the coefficients of the filter associated with the fixed equalizer are refreshed at regular intervals, resulting in a further improvement of the convergence of the clock of the receiving system with respect to that of the transmitting system.
The invention also proposes a receiving system which is characterized in that it comprises a means which enables the timing recovery device and the first equalizer to operate alternately during the second mode of operation.
Thus, in the second mode of operation, which corresponds to the data transmission phase, the timing recovery device and the first equalizer operate in turns, which allows a 50% saving on the calculation time to be achieved.
Finally, such a receiving system can be included advantageously in any digital communication system.